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Thomas R. Parish, David H. Bromwich, and Ren-Yow Tzeng

Abstract

The Antarctic topography and attendant katabatic wind regime appear to play a key role in the climate of the high southern latitudes. During the nonsummer months, persistent and often times intense katabatic winds occur in the lowest few hundred meters of the Antarctic atmosphere. These slope flows transport significant amounts of cold air northward and thereby modify the horizontal pressure field over the high southern latitudes. Three-year seasonal cycle numerical simulations using the NCAR Community Climate Model Version 1 (CCM1) with and without representation of the Antarctic orography were performed to explore the role of the elevated terrain and drainage flows on the distribution and evolution of the horizontal pressure field. The katabatic wind regime is an important part of a clearly defined mean meridional circulation in the high southern latitudes. The position and intensity of the attendant sea level low pressure belt appears to be tied to the Antarctic orography. The seasonal movement of mass in the high southern latitudes is therefore constrained by the presence of the Antarctic ice sheet. The semiannual oscillation of pressure over Antarctica and the high southern latitudes is well depicted in the CCMI only when the Antarctic orography is included.

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David L. Williamson, David H. Bromwich, and Ren-Yow Tzeng

Abstract

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David H. Bromwich, Ren-Yow Tzeng, and Thomas R. Parish

Abstract

The NCAR CCM1's simulation of the modern arctic climate is evaluated by comparing a five-year seasonal cycle simulation with the ECMWF global analyses. The sea level pressure (SLP), storm tracks, vertical cross section of height, 500-hPa height, total energy budget, and moisture budget are analyzed to investigate the biases in the simulated arctic climate.

The results show that the model simulates anomalously low SLP, too much storm activity, and anomalously strong baroclinicity to the west of Greenland and vice versa to the east of Greenland. This bias is mainly attributed to the model's topographic representation of Greenland. First, the broadened Greenland topography in the model distorts the path of cyclone waves over the North Atlantic Ocean. Second, the model oversimulates the ridge over Greenland, which intensifies its blocking effect and steers the cyclone waves clockwise around it and hence produces an artificial “circum-Greenland” trough. These biases are significantly alleviated when the horizontal resolution increases to T42.

Over the Arctic basin, the model simulates large amounts of low-level (stratus) clouds in winter and almost no stratus in summer, which is opposite to the observations. This bias is mainly due to the location of the simulated SLP features and the negative anomaly of storm activity, which prevent the transport of moisture into this region during summer but favor this transport in winter.

The moisture budget analysis shows that the model's net annual precipitation ([P - E]) between 70°N and the North Pole is 6.6 times larger than the observations and the model transports six times more moisture into this region. The bias in the advection term is attributed to the positive moisture fixer scheme and the distorted flow pattern. However, the excessive moisture transport into the Arctic basin does not solely result from the advection term. The contribution by the moisture fixer is as large as from advection. By contrast, the semi-Lagrangian transport scheme used in the CCM2 significantly improves the moisture simulation for this region; however, globally the error is as serious as for the positive moisture fixer scheme.

Finally, because the model has such serious problems in simulating the present arctic climate, its simulations of past and future climate change for this region are questionable.

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Tsing-Chang Chen, Ren-Yow Tzeng, and Ming-Cheng Yen

Abstract

The velocity-potential fields generated from the FGGE III-b horizontal winds of the European Centre for Medium Range Weather Forecasts were subjected to an empirical orthogonal function (EOF) analysis to extract the annual cycle and the 30–50 day mode of the divergent circulations. We found that the Indian monsoon circulation is portrayed by the annual cycle of the divergent circulation and develops as a classical, giant sea-breeze model. On the other hand, this monsoon system is modulated by the planetary-scale 30–50 day low-frequency mode to establish an onset-active-break-revival-retreat life cycle. This modulation is accomplished through the following interaction process. The northeastward propagation of the planetary-scale 30–50 day mode over the Indian monsoon region induces transient local Hadley circulation. Through this type of circulation, the planetary-scale 30–50 day mode couples with and steers northward the low-level, 30-50 day monsoon troughs and ridges that originated around the equator. The northward migration of these low-level transient troughs and ridges cause, respectively, the deepening and filling of the monsoon trough over central India. The evolution of this monsoon trough results in the intensification and weakening of the Indian monsoon and its life cycle.

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